Add a github action to monitor new tags, posts, resources

[ctmbl]

add a github action that parses .xml (for example at https://iscsc.fr/tags/index.xml) and compare it to the generated one on a PR and add a summary on the PR with:

• total number of each content
• new content (posts, tags, resources, author, series)
• closely related tags (to avoid duplication, ex: tool vs tools)
• removed content
• modified content

but also:

• warn if new content date is distant from today
• warn if new content is still in draft